Impact of variation in voluntary moderate deep inspiration breath hold on the 3D dose distribution in breast cancer radiotherapy

Aim Determination of impact of variations during voluntary moderately deep inspiration breath hold (vmDIBH) on doses to organs at risk (OAR) and target coverage in patients undergoing radiotherapy (RT) for left-sided breast cancer (BC).

Methods We recorded vmDIBH-breathing signals using a Surface Guidance system, without visual coaching, in 21 BC patients. In 13 patients, cone-beam CT (CBCT) scans were acquired with vmDIBH, immediately after treatment delivery. A 3D dose recalculation was performed on CBCT, cranio-caudally stitched with the planning CT for full dose evaluation. Mean Heart Dose (MHD), Mean Lung Dose (MLD) and V95% of the clinical target volume (CTV) were calculated. Relationships between vmDIBH level, stability and differences in Dose-Volume Histogram (DVH) parameters were analysed using linear regression and Mann-Whitney U-tests.

Results Of 150 recorded vmDIBHs, 39% of the breath-holds were within a predefined 3 mm gating window. The average deviation from this window was 0.6 mm (SD 1.9 mm), with the mean value just outside predefined boundaries. All recalculated CBCTs (n = 95) met predefined DVH criteria for MHD, MLD, and V95% of tumourbed CTV. The V95% of the whole breast CTV showed minor deviations from the planned dose (mean delta V95%: -0.8%, SD 1.6%). For patients with excursions beyond the 3 mm window, average differences were 1.43% in V95%-CTV, 0.02 Gy in MHD, and 0.05 Gy in MLD.

Conclusion Only small breathing excursions beyond 3 mm were observed during vmDIBH without visual coaching, not resulting in clinically relevant CTV Underdosage with MHD and MLD remaining within tolerance.